Our Planet

Basic Science of Tsunamis

Michael Sugarman and Devin Peters

What is a tsunami? Contrary to popular belief, a tsunami is not a single massively tall tidal wave (tsunamis are not related to the tides), but is in fact a succession of numerous shorter (in height) but very long waves comprised of enormous amounts of water. The actual word “tsunami” is derived from the Japanese words “tsu”, meaning harbor, and “nami”, which means waves (the figure below shows the two Japanese characters, tsu and nami ). The majority of tsunamis are the result of earthquakes beneath the ocean floor, while relatively few are caused by events such as landslides, volcanic eruptions and even meteoric impacts (these events are discussed further below).

What are the physical characteristics of a tsunami? Tsunami waves are characterized by long wavelengths with relatively short amplitudes (these terms are represented on the schematic figure in the following section). The height of tsunami waves (its amplitude) over the open water is typically less than one meter, but this gradually increases as it makes its way towards the shore. In fact, tsunamis often travel unnoticed under ships over the open ocean. In comparison, each wavelength is extremely long, reaching hundreds of kilometers. (Think of the wavelength as a measurement from the beach out to the horizon, not a measurement of the width of the wave from left to right with respect to the horizon.) When the tsunami waves reach the shore, their height may approach 30 meters as it is compressed by the rising ocean floor. The speeds with which these waves travel are impressive. The waves travel at between 500 to 1,000 km/h over the open water. These speeds decrease slightly as the tsunami waves approach and encounter resistance from the rising seabed close to shore.

Earthquakes are the most common cause of tsunamis. The earth's crust is comprised of numerous tectonic plates, some of which are beneath the ocean (please refer to the schematic below). When two separate plates come into contact, sometimes the heavier plate slips under the lighter one, in a process called subduction. Underwater subduction often leaves enormous deep ocean trenches within the seafloor. In some cases of subduction, the lighter plate may suddenly thrust upwards, thus causing an earthquake and generating seismic waves. When this plate jerks upward, the energy of that force is transferred to the water, pushing large volumes of seawater upward, above normal sea level; thus, a tsunami is born. The earthquake that generated the December 26, 2004, tsunami in the Indian Ocean measured 9.0 on the Richter scale, and was one of the largest earthquakes ever recorded.

What are the other potential causes of a tsunami? Although earthquakes cause the majority of tsunamis, there are other phenomena that can trigger a tsunami. These include events such as volcanic eruptions, landslides and even meteoric impacts (a meteor striking the ocean). The mass extinction of the dinosaurs and other animals that occurred 65 million years ago was caused by a meteorite impacting the ocean in the Gulf of Mexico. This impact resulted in an awesomely large tsunami of catastrophic proportions. There is evidence that the tsunami waves were approximately one kilometer (six-tenths of a mile) in height as they crashed ashore and surged deep inland, devastating everything in their path. It is interesting to note that there is debate over whether or not a tsunami could be intentionally generated by exploding a nuclear devise in the ocean, although most scientists agree this is unlikely, due to the immense energy required.

An historical perspective: A brief list of past major tsunamis. Below is a brief list of significant tsunamis that have hit across the globe within the past 150 years.

December 26, 2004 : A colossal earthquake in the Indian Ocean measuring magnitude 9.15 on the Richter scale triggered tsunami waves that annihilated coastlines in at least 10 different countries and caused an estimated 230,000 deaths. It is thus far the deadliest tsunami in recorded history.

These NASA photographs (taken by a Quickbird satellite) strikingly reveal the level of devastation wrought upon Banda Aceh, Indonesia by the December 26, 2004 tsunami. The satellite photographs show the city before and after the tsunami hit, respectively. (The left photo was taken on June 23, 2004, while the right photo was taken a few days after the tsunami, on December 28, 2004.)

Photos courtesy of NASA.gov

August 16, 1976 : A tsunami caused by an earthquake (7.9 on the Richter scale) hit the Moro Gulf region of the Philippines, killing an estimated 5,000 people and injuring close to 10,000 more.

May 22, 1960 : Tsunami waves up to 35 feet high struck Chile, Hawaii, the Philippines, Okinawa and Japan, killing over 1,000 people. This tsunami was caused by the most powerful earthquake ever recorded, named The Great Chilean Earthquake, it measured measuring an impressive 9.5 on the Richter scale.

January 31, 1906 : An offshore earthquake created a tsunami that swallowed part of Tumaco, Colombia. The death toll was estimated at between 500 and 1,500.

August 27, 1883 : The infamous volcano Krakatau erupted, creating a tsunami that swept over nearby Java and Sumatra, killing approximately 36,000 people.

Although tsunamis occur most frequently in the Pacific Ocean, due to the high volume of seismic activity, it is important to point out that tsunamis have the potential to strike nearly any coastline in the world.

What are the warning signs of an impending tsunami? It is possible that an earthquake will be felt, although it depends on the location and magnitude of that quake. Clues as to an approaching tsunami can be observed by the sometimes-odd behavior of the ocean at the beach. For instance, the ocean may appear as if it is boiling due to large quantities of gas that bubble to the water's surface. In addition, the water may become unusually warm and stink of sulfur (hydrogen sulfide), which closely resembles the odor of rotten eggs. Audible indicators may include an intense roaring bang, followed by a similar thunderous boom. As demonstrated at the beaches in Indonesia before the December 2004 tsunami hit, the tide may recede an unusually long distance out to sea. There is evidence to support the claim that a variety of animals sensed the imminent tsunami and sought safety further inland. In fact, strange animal behavior may be utilized by people as an effective early warning system, although their exact method of detection remains somewhat of a mystery.

How to prepare for a tsunami. If you suspect an impending tsunami while you are at (or near) the beach, as indicated by the warning signs mentioned above, there are certain precautions you can take to secure your safety. Quickly relocate inland from the water's edge to an elevated location. Once you reach an area of higher ground, listen to local radio or television stations for information from the Tsunami Warning Centers about further action. In some countries, there are Civil Defense Sirens that may sound before a tsunami hits. In addition, you should always carry a radio with you when you go to the beach and have fresh batteries in it.

References

National Geographic. Tsunamis

http://news.nationalgeographic.com/news/2004/12/1227_041226_tsunami.html

Physics Department, University of Washington.

http://www.ess.washington.edu/tsunami/general/physics/physics.html

How Stuff Works: Tsunamis.

http://science.howstuffworks.com/tsunami.htm

Wikipedia: The Free Online Encyclopedia

http://en.wikipedia.org/wiki/Tsunami

NASA.gov

http://rst.gsfc.nasa.gov/Sect3/Sect3_7.html

Earth Science Australia. The physics of a tsunami.

http://earthsci.org/education/teacher/basicgeol/tsumami/tsunami.html

About.com. How to prepare for a tsunami.

http://geography.about.com/od/physicalgeography/a/tsunami.htm

Columbia University. Scientific data on the December 2004 Tsunami.

http://iri.columbia.edu/~lareef/tsunami/

National Oceanic and Atmospheric Administration

http://nctr.pmel.noaa.gov/faq_display.php?kw=1998%20Interview%20with%20Dr.%20Frank%20Gonzalez#6